The Peterborough-based agency, which is funded by the Department for the Environment, Food and Rural Affairs, mainly operates Vauxhall Vectras, Astras and Corsas under a solus badge deal.
It said it is switching to LPG because it feels that gas power is more in keeping with its green transport policy as the vehicles produce less harmful emissions.
Fleet manager Marita Johnson said the watchdog's drivers are happy to switch to LPG after driving demonstrators supplied by Vauxhall.
She said: 'We have had a number of demonstrators at the Peterborough head office to enable me to organise test drivers for my drivers, and after driving the cars and realising there is no loss of performance, they have all been happy.'
The agency has ordered 1.6-litre 16v LPG-powered LS Astras, both estate and hatchback, and has just taken delivery of the first batch of 20 vehicles.
It will also order gas-powered Vectras and Zafiras and predicts it will take two years before the whole fleet has switched to gas power.
The agency said it expects to make savings on fuel costs using LPG-powered vehicles but that it has not calculated the amount, stating that 'cost has come a distant second to environmental concerns'.
However, Vauxhall calculates that a 25-strong fleet of vehicles travelling about 25,000 miles a year and using LPG for 90% of the time would save an operator more than £30,000 over three years.
English Nature operates its cars on a pool system with all vehicles parked up at the regional offices overnight. No employee is permitted to take the vehicles home unless they have a business appointment the next morning.
It also operates an amount of vehicles for site managers and other nature reserve-based workers and these consist mainly of Land Rover and other off-road vehicles.